When switching inductive loads using power transistors, the requirement often exists to limit the inductive voltage on the power transistor, or that anywhere on the electronic circuit, to a defined value. For this purpose, a protection element can be connected in a known manner in parallel with the emitter-collector junction of the output transistor. Furthermore, European No. 0,174,473 discloses the connection of a zener diode in parallel with the collector-base junction of the output transistor, which zener diode raises the base of the output transistor on reaching an upper limiting voltage of, in general, over 250 volts.
In specific applications, the requirement furthermore exists to be able to carry out voltage limiting at a second, lower voltage level as well, in order, for example, to be able to reduce the stored energy in an ignition coil to this lower voltage level so that no ignition sparks can occur. This so-called spark-free disconnection is fundamentally necessary when the disconnection of the output stage is not intended to be used for producing an ignition spark. In order to create such voltage limiting at a lower voltage level, in the case of the above-mentioned prior art, a voltage divider is connected in parallel with the collector-emitter junction of the output transistor, a pick-off acting via transistor stages on the base of a preliminary-control transistor. Furthermore, an external switching transistor is connected between the base of the preliminary-control transistor and earth in order to predetermine the disconnection mode. Only when this switching transistor is switched off can the base of the preliminary-control transistor be raised to the voltage limit via the voltage divider and the transistor circuit. A large number of components are required for this purpose in the case of the known circuit, which means a cost outlay which is not inconsiderable.